Bannon acknowledged that if James Comey, above, had not been ousted from the FBI, special counsel Robert Mueller's investigation into Russian meddling in the 2016 campaign and possible collusion with the Trump campaign would not have "the breadth" that it has now. (AP Photo/Alex Brandon)

Former White House chief strategist Steve Bannon split from President Trump by saying Trump's decision to fire ex-FBI Director James Comey was possibly the biggest mistake in modern political history.

"That probably would be too bombastic, even for me," Bannon told CBS's Charlie Rose when asked about whether Bannon thought Trump's dismissal of Comey in July was the biggest mistake in political history. "But maybe modern political history," he added.

Bannon's comments were recorded for a "60 Minutes" that aired Sunday. It was billed as the first-ever television interview for the newly reinstated executive chairman of Breitbart News. He departed the Trump administration in August, about one year after he joined the Trump campaign as chief executive officer.

While refusing to comment on private conversations in the Oval Office, Bannon said he was a "big believer" in Washington, D.C., as "a city of institutions and not individuals."

"With [Senate Majority Leader Mitch] McConnell and [House Speaker Paul Ryan], those institutions can be changed if the leadership is changed," Bannon said on why his plans to go to war with the Republican establishment was different from axing Comey. "I don't believe that the institutional logic of the FBI, and particularly in regards to an investigation, could possibly be changed by changing out the head of it."

Bannon acknowledged that if Comey had not been ousted, special counsel Robert Mueller's investigation into Russian meddling in the 2016 campaign and possible collusion with the Trump campaign would not have "the breadth" that it has now.

Trump revealed in an NBC News interview in May that he fired Comey, whose agency had already begun an investigation into Russian interference, while considering "this Russia thing." While Comey had privately assured Trump he was not personally under investigation while Comey was in charge, that changed shortly after Comey was fired, which prompted an expanded probe looking for possible obstruction of justice, according to a Washington Post report in June.

"I don't think there's any doubt that if James Comey had not been fired, we would not have a special counsel," Bannon added before denying there had been internal conversations in the Trump administration about firing Mueller.